The present invention relates to a manually guided implement, such as a chainsaw, a brush cutter, a trimmer, a cut-off machine, or the like, and has an internal combustion engine and a carburetor for supplying a fuel/air mixture to the engine, wherein a stream of air for combustion is drawn through the carburetor via the engine.
Manually guided implements having an internal combustion engine as the drive motor are operated under changing climatic conditions. In this connection, the internal combustion engine draws in air for combustion from the atmosphere through the carburetor to form a fuel/air mixture. The drawn-in air for combustion is, of course, influenced by the environmental conditions, especially with regard to its temperature and moisture content.
Within the carburetor, the stream of air for combustion has a relatively low speed, which is accompanied by a drop in pressure, relative to the atmospheric pressure. In a narrowed cross-sectional portion of the carburetor, for example in the form of a venturi section, a further drop in pressure in the stream of air for combustion is produced for the drawing-in of the fuel through a fuel nozzle. The temperature drop that accompanies the drop in pressure in the combustion air stream, and in particular the temperature drop that is caused by the vaporization enthalpy of the fuel that is volatilized in the carburetor, can lead to a condensation of moisture in the air. At lower temperatures, especially around the freezing point, the condensed moisture can freeze up or ice over the carburetor in the region of the fuel nozzle, which adversely affects the formation of the mixture. The result is an unsteady running of the engine or even a stopping of the engine. Icing over is particularly detrimental on the butterfly valve and on its seat in the venturi section, since as a result the air gap during idling is closed off. This causes the engine to die during idling.
Various embodiments of manually guided implements are known that are provided with means for preventing icing over of the carburetor. For example, the carburetor can if necessary be heated with electrical heating wires, whereby, however, the technical conversion is complicated and hence expensive. The air for combustion can also be preheated, for which purpose it is guided past hot portions of the internal combustion engine. During the intake process through the carburetor, the heated combustion air has an adequate excess temperature to avoid condensation of water and hence an icing over. However, a drawback of this arrangement is the increased complexity of the combustion air guidance, along with corresponding output-retarding flow resistance and the increase of the air moisture due to evaporating drops and melting ice. A further drawback is in the reduction of the air density, which can similarly cause a drop in output of the internal combustion engine.
It is therefore an object of the present invention to reliably prevent icing over of the carburetor of a manually guided implement.